The invention relates to a piston engine comprising at least one piston and at least one cylinder made of light alloy and having a chrome-plated running friction surface.
Such piston engines are known, for example from the auto magazine article xe2x80x9cWeitere Fortschritte mit verchromten Leichtmetallzylindernxe2x80x9d [Further Progress made with chrome-plated light-alloy cylinders] by Dr. E. Mahle published in Motorzeitung (MZT), Year 1953, page 63.
It has been found recently that such a pairing of the running friction surface of pistons in the development of engines designed accordingly in the direction of higher power outputs, which are frequently employed in mopeds, lawnmowers or saws, are no longer adequately safe with respect to resistance to seizure. It was found that the cause of this are increased lateral forces and a transfer of material from the chrome-plated running surface of the cylinder into the micro-range of uncoated pistons resulting from such forces.
As a remedy, provision has been made first for a synthetic resin-graphite coating of the running surface. However, such a coating showed increased wear as compared to the use of conventional running surface pairings.
It has been admittedly possible to substantially reduce such wear by minimizing the roughness of the chromium coating; however, said measure also leads to burnt piston rings.
The invention, therefore, is dealing with the problem of reducing in connection with piston engines comprising at least one cylinder made of light alloy and having a chrome-plated running surface the tendency to seizure occurring between the piston and the cylinder.
Said problem is solved by the characterizing feature of claim 1. Advantageous further developments of the invention are the objects of the dependent claims.
The desired effect has been achieved solely by the use of synthetic resin coatings as defined by the invention, which contain hard particles incorporated in the coating. This is ascribed to the fact that the coating as defined by the invention creates quasi by itself a favorable counter running surface by means of the embedded hard particles in that the chromium layer is xe2x80x9cgroundxe2x80x9d. Even small amounts of hard particles, for example about 5 percent by weight, preferably less than 3 percent by weight suffice for obtaining the effect as defined by the invention. Even a component amounting to only 1% can produce the desired effect in most cases. The preferred mean diameter of the hard particles comes to 3 to 7 xcexcm.
The coating is produced by screen printing, ink ball printing or by spraying.
It is known per se from DE 3047978 A to coat the running surface of a piston and/or of a cylinder for special piston compressors with a lacquer produced on the basis of epoxy resin with an oxide-ceramic material added to said epoxy resin. However, said coating has been disclosed exclusively as an alternative to the known iron, chromium or nickel coatings, which are described as being relatively costly and to some extent difficult to produce in terms of processing technology. It is finally stated in said cited reference that only one of the two friction partners is coated, and that the piston offers itself on account of the lower work expenditure.